Future<void> main() async {
  print(checkVersion());
  var a=await checkVersion();
 print(a);
  var numbers = countUpTo(5);
  print(numbers); // (1, 2, 3, 4, 5)
}

 Future<String> lookUpVersion() async {
  return '1.0.1';
}

Future checkVersion() async {
  var version=await lookUpVersion();
  print(version);
  return 1;
}

// 返回 Iterable
Iterable<int> countUpTo(int max) sync* {
  for (int i = 1; i <= max; i++) {
    yield i;
  }
}

/*可调用类*/


